What if my dog gets tired halfway through?
It happens, and it's manageable. Smaller dogs can simply be carried for a stretch, and on some routes — Annapurna especially — there's jeep access at various points if you need to get a tired dog off their feet for a while. In the worst case, a helicopter is possible, just as it would be for a person who got sick, though it's expensive and very much a last resort.
The better approach is not to let it get that far: keep an eye on your dog rather than just pushing on regardless. If they're looking properly worn out, or you spot a blister, take a rest day or two, or swap a full day for two half days. It's rarely worth the trade-off of forcing on when they need a break.